Right, as you helpfully pointed out last week in that other thread and I haven't forgotten. It supports the case for going sealed here. We would/could tweak the box volume/QTC for the sealed boxes so the -3 db point falls right at 80 hz. That I think is the simplest/cheapest way to go.

However, if blending with flanking subs ala Parham, then one might want a higher crossover with a lower slope and would pay the cost of smoother bass in higher system complexity, happily in my case if those upper-bass nulls would go away.

I still don't see the attraction of ported (which again makes me wonder what I'm missing since ported seems so popular) unless one wants to get enough bass to be usable for music w/o a sub. That I think takes a little more xmax than the drivers we've been talking about, or sacrificing some SPL.

apologies if i was repeating jack. i can't keep all the threads in my head at once. :-)

"I still don't see the attraction of ported (which again makes me wonder what I'm missing since ported seems so popular) unless one wants to get enough bass to be usable for music w/o a sub. That I think takes a little more xmax than the drivers we've been talking about, or sacrificing some SPL."

some like to run the mains lower without subs. that is reason one.

others like to "stretch" the high sensitivity drivers that rolloff in sealed much higher than 80hz down to at least 80hz. that is reason two.

there may be other reasons, such as limiting cone excursion and the positive effect that has on excursion related distortion...

for the dayton and the deltalite ii 2512, sealed would seem to be a no-brainer when employing subs.

Sorry if this is a bit too OT... is there something fundamentally unique about "flanking subs" that makes them very different from what many people call "bass bins", or even simply designing a large full-range speaker (2.5- or 3-way) in the first place, in a single cabinet?

I see Wayne Parham recommends particular placement (beneath, beside and slightly behind) - is this an important part of the recipe, or just the most sensible suggested placement assuming discrete sub boxes?

I'm a big 2-channel music guy, so I suspect I will be happiest if I can get a substantial portion of the midbass and bass from in the vicinity of my mains - with mode smoothing tackled after that with two or more additional smaller subs elsewhere in the room.

anti: flankers are purposely distanced from the mains by a couple feet so that the nulls created by boundary reflections from them are at different frequencies than the nulls created by boundary reflections from the mains. Then, in range of frequencies where both have output, one fills in the nulls for the other. You don't get enough separation with sub and mid woofer in same cabinet to get the same effect.

anti: flankers are purposely distanced from the mains by a couple feet so that the nulls created by boundary reflections from them are at different frequencies than the nulls created by boundary reflections from the mains. Then, in range of frequencies where both have output, one fills in the nulls for the other. You don't get enough separation with sub and mid woofer in same cabinet to get the same effect.

Thanks for the clear and concise explanation Jack, makes tons of sense.

anti: flankers are purposely distanced from the mains by a couple feet so that the nulls created by boundary reflections from them are at different frequencies than the nulls created by boundary reflections from the mains. Then, in range of frequencies where both have output, one fills in the nulls for the other. You don't get enough separation with sub and mid woofer in same cabinet to get the same effect.

That could be applied to a TWW, with a box of suitable depth, by placing the second woofer on the back panel.

Quote:

Originally Posted by JackNC

anti: flankers are purposely distanced from the mains by a couple feet so that the nulls created by boundary reflections from them are at different frequencies than the nulls created by boundary reflections from the mains. Then, in range of frequencies where both have output, one fills in the nulls for the other. You don't get enough separation with sub and mid woofer in same cabinet to get the same effect.

the thx spec for low pass sub is 4th order. it assumes a flat frequency response through the crossover region in order to match to the 4th order, net, highpass from the mains.

as for how it rolls off at the very bottom end, thx spec is for sealed enclosures that take advantage of pressure vessel gain.

all that said, even most commercial theaters don't adhere to the specs, so its a crapshoot.

"Isnt this why most speakers are designed to have -3db points at least half an octave away from the intended crossover point?"

no. the -3db point is where you should, in theory, be crossing to the subs. a 2nd order cross is also -3db, so combined the system will be down 6db at the crossover point. when combined with subs that are also -6db at the crossover point, the net frequency response will be flat.

the thx spec for low pass sub is 4th order. it assumes a flat frequency response through the crossover region in order to match to the 4th order, net, highpass from the mains.

as for how it rolls off at the very bottom end, thx spec is for sealed enclosures that take advantage of pressure vessel gain.

all that said, even most commercial theaters don't adhere to the specs, so its a crapshoot.

"Isnt this why most speakers are designed to have -3db points at least half an octave away from the intended crossover point?"

no. the -3db point is where you should, in theory, be crossing to the subs. a 2nd order cross is also -3db, so combined the system will be down 6db at the crossover point. when combined with subs that are also -6db at the crossover point, the net frequency response will be flat.

Ok, that makes sense i wasnt aware the crossover slopes were different...... Ideally we want them to be the same.

bwaslo: "if they could be mounted up against the ceiling, woofers at the top of the baffle near the ceiling to avoid reflection (ground-plane style)"

You could put 'Malcom in the middle' of your driveway, on the ground, woofers down, to simulate the first bounce of the proposed ceiling mounting.

Wondering, would the ceiling affect the vertical polar?

Also wondering if instead of waveguide above Ms,M,M,Ms--- could the vertical polar be improved by Ms, T, Ms over M,M (Ms being the shaded speakers) or viceversa for the ceiling install.................

I hate to come off all anti-theoretical, but the response of speakers at bass and sub frequencies in a room has so tremendously little to do with slopes and filter shapes that the are hardly worth discussing. You're measuring with a micrometer and cutting with an axe, here. Measure an In-room response of a speaker and compare to near field at bass freqs to see what I mean!

I hate to come off all anti-theoretical, but the response of speakers at bass and sub frequencies in a room has so tremendously little to do with slopes and filter shapes that the are hardly worth discussing.

True, but if there's a phase cancellation because of a poor XO it's an independent issue, right?

Bill, perhaps you can answer a question that I've posed to others a few times over the years but never gets answered:

Taking a XO not swamped by room effects, say W-T, is it really important what the slopes are once either driver is down by, say, 6 dB?

At that point, the rolling off driver can only add or subtract (depending on phase) 1 dB from the overall output at that freq.

I know you asked Bill, but I'd say it depends on the cross over slope mostly. If it's a 2nd order, yes it still matters a great deal even below the -6db mark. There is still quite a bit of interaction. If 8th order, then once past -6db, there quickly becomes hardly any influence, so it would matter very little.

All in all, I feel the knee of the slope is the most important to get right. A lot happens right there. Phase, amplitude, and shape are all important. I argued Wolf over at Tech Talk about this once. He was claiming a bunch of his designs used odd order cross overs. They did, but in all of his examples, the -6db and above areas were essentially even order LR4. So I said, sure they're odd order below the knee, where it's not a big deal, but those are LR4 cross overs.

Because of the fixed short distance, bipole path cancellation occurs at a fixed higher frequency...opening a new can of worms. Adjusting flanker position for complementary in-room response seems more realistic.

I know you asked Bill, but I'd say it depends on the cross over slope mostly. If it's a 2nd order, yes it still matters a great deal even below the -6db mark. There is still quite a bit of interaction.

Powers don't necessarily add like that. For example, if you place two identical drivers next to each other you gain more than 3 dB (2x power). It's closer to 6 dB, which would be summing of the pressures (20log10(2)~6). If you're wondering how to reconcile this with the power viewpoint, what happens is the system's radiation impedance changes.

If you sum the pressures of a source at 0 dB and a source at -6 dB, that is 1.5x or .5x the pressure, which is +3.5 dB or -6dB. To see how far down the lower source has to be in order to get the deviation below 1 dB, set -1 = 20log10(1-p_low) and solve to get p_low ~.1 or -20 dB.

The purpose of using a waveguide of that size and a compression driver is that they can work down to 1 kHz or even lower. 6.4k+ could be covered by a .5" dome tweeter on a small waveguide. To put this in perspective, the SEOS-18 can be used from about 800-20k Hz. 800-6.4k is 3 octaves. 6.4k-20k is a little over 1.5 octaves, and there is less power per octave at those frequencies in most music.

The midranges don't really match up with the rest of the system in terms of directivity, sensitivity, or power handling. You could do them next to each other horizontally; that would sort of match the horizontal directivity of the horn but you'd have to cross a little higher than desired and the vertical would be too wide. Also, when you turn the stereo up, the 1" coils on the mids will heat up much faster than the other drivers, causing a dip in the middle of the overall response. If you don't listen very loud this might not be a huge problem but it's something to consider.

First order crossovers are generally a bad idea. Even if they sum perfectly on-axis, when you go off-axis vertically the path length differences to the drivers causes a relative phase shift which makes them sum differently. That effect is present with every crossover, but the shallow roll-off of 1st orders makes it a problem over a large range of frequencies. So even if the response is flat on axis, if you move up or down you get huge holes in the response. Even if you only listen in front of the speaker, what you hear is also shaped by later arriving reflected sound that may have originally emanated from off-axis.

My suggestion would be to simplify. Do the SEOS-18 + CP380M and a single TD15H. Cross them around 1 kHz, maybe slightly lower, with 4th or 8th order slopes. If you don't have a lot of experience with passive crossovers, you're probably better off going active. This is a setup that, properly tuned, can give you first rate sound quality and serious output. Adding a midrange in-between these doesn't really provide a lot of benefit and it makes the system harder to tune and more costly.